The Exploratorium is a huge hands-on science museum in San Francisco. The museum also has an extensive outreach program to the world's science teachers. At the Exploratorium booth, the staff demonstrated several simple experiments that help students learn different scientific principles.
One apparatus, made of a plastic tube, a 500-turn coil of wire and a few disk magnets, creates a simple generator when you shake the magnets back and forth in the tube:
This is the simplest possible electrical generator. A two-color light emitting diode (LED) mounted on the tube showed that the apparatus was generating current.
Another apparatus modulates a radio's sound onto the light from an LED, which is then turned back into a electrical signal by a solar cell:
Simple, inexpensive setups like these are called "snacks," and you can see dozens of them at www.exploratorium.org/snacks.